1. Field of the Invention
The present invention relates to an amplification circuit which amplifies a high-frequency signal, and, more particularly to a wide-band high-frequency amplification circuit which amplifies a signal over a wide frequency band. The high-frequency amplification circuit according to the present invention is adaptable as a wide-band amplification circuit to both wireless communication and cable communication which demand a high transfer rate.
2. Description of the Related Art
Recently, wide-band communication is needed in wireless communication and wide-band communication of 54 Mbps is made into practical use according to the IEEE 802.11a standards. Further, standardization of an ultra wide band (UWB) which is for wireless communication of 1 Gbps class is being made in the IEEE 802.15.TG3a standards. In such wireless communication, the occupied frequency band that is needed for wireless transmission becomes very wide as apparent from the Shannon's theorem. For example, a wide frequency band of 3.1 GHz to 10.6 GHz is used in UWB (see, for example, “Nikkei Electronics”, Issue of Mar. 31, 2003 (pp 30 to 31)). However, there has never been a wide-band wireless communication at a frequency approximately three times the lower limit frequency, i.e., over three octaves.
FIG. 25 is a timing chart for a UWB wireless system. In a UWB wireless system called a multi-band system, for example, frequency band ranging from 3.1 GHz to 10.6 GHz are separated into, for example, eight frequency bands each having a band of 500 MHz or so, as shown in FIG. 25, and a wide band is covered by hopping the eight frequency bands sequentially at a high speed.
A resistance matching amplification circuit, a negative feedback amplification circuit, a traveling-wave amplification circuit and so forth are known as a circuit which amplifies a wide-frequency signal (see, for example, “Microwave Transistor” by Yoichiro Takayama, IEICE (pp 164 to 174 and 184 to 192)). However, the resistance matching amplification circuit should face a problem of an increased noise factor (NF), the negative feedback amplification circuit should face a problem of an increase in NF and gain reduction, and the traveling-wave amplification circuit should face a problem that the size of the distribution line would increase.
By way of contrast, a so-called gate-grounded circuit in which the gate of an MOSFET is grounded or a so-called base-grounded circuit in which the based of a bipolar transistor is grounded has an advantage such that input matching is easily taken (see, for example, “A 1 GHz CMOS RF front-end IC for a direct-conversion wireless receiver” by A. Rofougaran, IEEE Journal of Solid-State Circuits (Volume 31 Issue 7, July 1996, pp 880 to 889).
There is also an amplification circuit which is given a band-pass characteristics by setting an inductor and a capacitor as loads of a transistor, as shown in FIG. 26 (see, for example, “A front-end filter with automatic center frequency tuning circuitry” by Y. Chang, Southwest Symposium on Mixed-Signal Design (2001, pp. 28 to 31). This example has a structure such that a varactor is used as a capacitor C1 whose capacitance is changed to match the center frequency of the band-pass characteristic chip by chip in accordance with a change in center frequency due to a process variation.
When a signal in a wide band ranging from 3.1 GHz to 10.6 GHz is amplified with low noise, however, the resistance matching amplification circuit, the negative feedback amplification circuit and the traveling-wave amplification circuit have the problems of an increase in NF, gain reduction and an increase in size.
While the gate-grounded circuit and the base-grounded circuit have an advantage of easy input matching, the advantage does not directly lead to adaptation of the load characteristics and transistor characteristics to a wide band.
Further, the example in FIG. 26 copes with a process-originated variation in band-pass center frequency, but neither aims at adapting the band-pass center frequency to a wide band nor has a structure designed to cope with the adaptation of the band-pass center frequency to a wide band.